Overview

Trojan:MSIL/Perseus is a sophisticated Remote Access Trojan (RAT) and credential stealer compiled entirely in the .NET framework, designed for deep endpoint surveillance and data exfiltration.

What is MSILPerseus?
For the victim, Perseus operates silently, granting a remote attacker full access to their file system, webcam, and keystrokes. For incident responders, Perseus is a modern, modular .NET threat. Because it relies on the MSIL (Microsoft Intermediate Language) architecture, it is frequently packed with commercial protectors (like ConfuserEx) to heavily obfuscate its code, making static analysis and reverse engineering significantly more difficult than traditional C++ RATs.

Infection Vectors & Threat Hunting
Perseus is typically distributed via weaponized spam campaigns containing malicious Office documents or dropped by initial access brokers. Upon execution, the .NET payload drops a copy of itself into the %LocalAppData% directory and establishes persistence via a scheduled task or Registry Run key. It utilizes dynamic API resolution to evade EDR hooks and frequently injects its core module into legitimate .NET processes (like RegAsm.exe or MSBuild.exe) to mask its network activity.

Forensic Analysis & Impact
The impact of Perseus is total system compromise. It actively hunts for and steals cryptocurrency wallets, browser cookies, and saved FTP credentials. Incident responders should monitor for anomalous outbound TCP traffic to dynamic DNS providers originating from native Windows binaries. Memory forensics is critical; analysts must dump the injected process memory to extract the un-obfuscated .NET assembly, which can then be decompiled (using dnSpy) to reveal the C2 configuration and encryption keys.

MITRE ATT&CK Techniques

Observed techniques used by this family, mapped to the MITRE ATT&CK framework:

Technique	Name	Tactic
`T1055`	Process Injection	Defense Evasion
`T1056.001`	Input Capture: Keylogging	Collection
`T1555`	Credentials from Password Stores	Credential Access
`T1027.002`	Obfuscated Files or Information: Software Packing	Defense Evasion
`T1571`	Non-Standard Port	Command and Control

Generated Detections (Boilerplate)

These YARA and Sigma rules are auto-generated based on the family name and aliases. They must be heavily tuned before deployment in a production environment.

YARA Rule

rule MALWARE_WIN_MSILPERSEUS {
    meta:
        description = "Detects Msilperseus (trojan_generic)"
        author = "SystemHelpdesk Boilerplate Generator"
        date = "2026-07-06"
    strings:
        $s1 = "msilperseus" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

Sigma Rule

title: Suspicious Msilperseus Activity
id: 3abb4f0f18ea726d7bfaff625db9a8fa
status: experimental
description: Detects generic indicators of the msilperseus malware family.
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
        CommandLine|contains:
            - "*msilperseus*"
    condition: selection
level: medium

Containment & Response Steps

Ordered checklist for responders. Adapt to your environment and engage professional support for active incidents.

Immediately isolate the endpoint from the network to sever the attacker's interactive, remote-control session.
Capture a full RAM image of the endpoint; memory forensics is critical for extracting the un-obfuscated .NET Perseus configuration.
Decompile the extracted .NET payload to identify the specific C2 IP address/domain and implement immediate blocks at the perimeter firewall.
Assume total endpoint compromise; perform a clean OS rebuild and force password resets for all accounts that accessed the machine.

What to Avoid

Common mistakes during response to this family that can destroy evidence, spread the infection, or worsen recovery.

Do not leave the machine connected to the network during triage; the attacker has live access and will likely destroy evidence or deploy ransomware.
Avoid relying solely on static file analysis, as the heavy .NET obfuscation (ConfuserEx) will yield minimal intelligence without memory dumping.

Msilperseus

Overview

MITRE ATT&CK Techniques

Generated Detections (Boilerplate)

YARA Rule

Sigma Rule

Containment & Response Steps

What to Avoid

References & External Analysis

Related Families (Category: trojan_generic)

Machine-readable

Ecosystem & Interactive Environments